Conventionally, as a material of an automotive outer panel which is required to exhibit excellent dent resistance such as a hood, a door, a trunk lid, a back door or a fender, a BH steel sheet of TS:340 MPa class (baking hardening type steel sheet, hereinafter simply referred to as 340BH) has been used. 340BH is ferrite single-phase steel where a solid solution C content in extremely low carbon steel having less than 0.01 mass % C is controlled with the addition of a carbide or nitride forming element such as Nb or Ti, and solid solution strengthening is performed by Si, Mn or P. In recent years, from the further demand for the weight reduction of automotive bodies, various investigations have been carried out to increase the strength of the exposure panels to which the 340BH has been applied, to achieve a reduction in the thickness of the steel sheet, a weight reduction in the R/F (Reinforcement: inner reinforcement parts) with the same thickness of outer panels, a reduction in the temperature and the time in a bake coating process, and the like.
However, if large amounts of Si, Mn, and P are further added to the conventional 340BH in order to increase the strength thereof, surface distortion of pressed parts considerably deteriorates due to the increase in YP. In this case, the surface distortion indicates minute wrinkles and/or undulated patterns which are likely to occur on a press-formed surface, such as a periphery of a door knob portion. Since the surface distortion remarkably degrades the appearance quality of automobiles, steel sheet applied to the exposure panels is required to have low yield stress before press forming which is close to YP of the present 340BH while the strength of a press-formed product is increased. In the same manner, when TS of a material changes, the amount of spring back of the pressed parts change thus inducing the generation of the surface distortion. Accordingly, to decrease the surface distortion, it is also necessary to decrease a change in TS in a coil in addition to the suppression of YS at a low value.
Further, a steel sheet used in such an application is also required to exhibit excellent stretch-formability and high ductility (El) in a coil stably.
Further, such a steel sheet used for manufacturing an automobile body outer panel part is also required to exhibit excellent corrosion resistance. That is, when a dent or a scratch is formed on a steel sheet by a stone scattered during traveling of an automobile, rust is liable to occur in such a portion and become a cause of forming a hole in the portion. To suppress such corrosion, it is necessary for the steel sheet to exhibit the excellent chipping resistance equal to or more than the chipping resistance of conventional 340BH.
Under such a background, for example, patent document 1 discloses a method of acquiring a galvannealed steel sheet which exhibits both low yield stress (YP) and high ductility (El) by forming the composite microstructure mainly constituted of ferrite and martensite by properly setting a cooling rate after annealing steel containing 0.005 to 0.15% C, 0.3 to 2.0% Mn and 0.023 to 0.8% Cr.
Patent document 2 discloses a method of acquiring a steel sheet which exhibits excellent ductility and high BH by containing by mass % 0.005 to 0.04% C, 1.0 to 2.0% Mn, 0.10% or less P, 0.03% or less S, 0.01 to 0.1% Al, less than 0.008% N, and 0.2 to 1.0% Cr while satisfying Mn+1.29Cr being 2.1 to 2.8, and by having the microstructure comprising ferrite and a 3.0% or more and less than 10% of martensite in a volume fraction.
Further, patent document 3 discloses the acquisition of a steel sheet which exhibits YP of 300 MPa or less thus exhibiting excellent ductility (El) and stretch-flangeability (hole expanding ratio, λ) by setting a sum of contents of Mn, Cr and Mo in a steel containing by mass % 0.02 to 0.033% C, 1.5 to 2.5% Mn, 0.03 to 0.5% Cr and 0 to 0.5% Mo to 1.8 to 2.5%.
Patent document 4 discloses the acquisition of a high strength cold rolled steel sheet which is excellent in baking hardening property and room-temperature aging resistance by controlling a composition range of C and Mn to C+Mn/20≧0.12% in a steel containing by mass % 0.03 to 0.09% C, 1.0 to 2.0% Mn, 0.005 to 0.1% sol. Al, 0.001 to 0.003% B.
Further, patent document 5 discloses a technique which decreases a change in material in a coil by decreasing a precipitation quantity of fine AlN by controlling a composition range of N and B to N−14/11B≦10 (ppm) in a cold rolled mild steel sheet containing by weight % C≦0.05%, Mn≦0.5%, N≦0.005% and B≦0.005%.